IPv6 maintenance Working Group (6man) F. Gont
Internet-Draft SI6 Networks / UTN-FRH
Intended status: Standards Track March 21, 2013
Expires: September 22, 2013
A method for Generating Stable Privacy-Enhanced Addresses with IPv6
Stateless Address Autoconfiguration (SLAAC)
draft-ietf-6man-stable-privacy-addresses-04
Abstract
This document specifies a method for generating IPv6 Interface
Identifiers to be used with IPv6 Stateless Address Autoconfiguration
(SLAAC), such that addresses configured using this method are stable
within each subnet, but the Interface Identifier changes when hosts
move from one network to another. The aforementioned method is meant
to be an alternative to generating Interface Identifiers based on
IEEE identifiers, such that the benefits of stable addresses can be
achieved without sacrificing the privacy of users.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 22, 2013.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
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to this document. Code Components extracted from this document must
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Design goals . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Algorithm specification . . . . . . . . . . . . . . . . . . . 7
4. Resolving Duplicate Address Detection (DAD) conflicts . . . . 10
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
6. Security Considerations . . . . . . . . . . . . . . . . . . . 12
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
8.1. Normative References . . . . . . . . . . . . . . . . . . . 14
8.2. Informative References . . . . . . . . . . . . . . . . . . 14
Appendix A. Privacy issues still present with RFC 4941 . . . . . 16
A.1. Host tracking . . . . . . . . . . . . . . . . . . . . . . 16
A.1.1. Tracking hosts across networks #1 . . . . . . . . . . 16
A.1.2. Tracking hosts across networks #2 . . . . . . . . . . 16
A.1.3. Revealing the identity of devices performing
server-like functions . . . . . . . . . . . . . . . . 17
A.2. Address scanning attacks . . . . . . . . . . . . . . . . . 17
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 18
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1. Introduction
[RFC4862] specifies the Stateless Address Autoconfiguration (SLAAC)
for IPv6 [RFC2460], which typically results in hosts configuring one
or more "stable" addresses composed of a network prefix advertised by
a local router, and an Interface Identifier (IID) that typically
embeds a hardware address (e.g., using IEEE identifiers) [RFC4291].
Generally, stable addresses are thought to simplify network
management, since they simplify Access Control Lists (ACLs) and
logging. However, since IEEE identifiers are typically globally
unique, the resulting IPv6 addresses can be leveraged to track and
correlate the activity of a node over time and across multiple
subnets and networks, thus negatively affecting the privacy of users.
The "Privacy Extensions for Stateless Address Autoconfiguration in
IPv6" [RFC4941] were introduced to complicate the task of
eavesdroppers and other information collectors to correlate the
activities of a node, and basically result in temporary (and random)
Interface Identifiers that are typically more difficult to leverage
than those based on IEEE identifiers. When privacy extensions are
enabled, "privacy addresses" are employed for "outgoing
communications", while the traditional IPv6 addresses based on IEEE
identifiers are still used for "server" functions (i.e., receiving
incoming connections).
As noted in [RFC4941], "anytime a fixed identifier is used in
multiple contexts, it becomes possible to correlate seemingly
unrelated activity using this identifier". Therefore, since
"privacy addresses" [RFC4941] do not eliminate the use of fixed
identifiers for server-like functions, they only *partially*
mitigate the correlation of host activities (see Appendix A for
some example attacks that are still possible with privacy
addresses). Therefore, it is vital that the privacy
characteristics of "stable" addresses are improved such that the
ability of an attacker correlating host activities across networks
is reduced.
Another important aspect not mitigated by "Privacy Addresses"
[RFC4941] is that of host scanning. Since IPv6 addresses that
embed IEEE identifiers have specific patterns, an attacker could
leverage such patterns to greatly reduce the search space for
"live" hosts. Since "privacy addresses" do not eliminate the use
of IPv6 addresses that embed IEEE identifiers, host scanning
attacks are still feasible even if "privacy extensions" are
employed [Gont-DEEPSEC2011] [CPNI-IPv6]. This is yet another
motivation to improve the privacy characteristics of "stable"
addresses (currently embedding IEEE identifiers).
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Privacy/temporary addresses can be challenging in a number of areas.
For example, from a network-management point of view, they tend to
increase the complexity of event logging, trouble-shooting, and
enforcing access controls and quality of service, etc. As a result,
some organizations disable the use of privacy addresses even at the
expense of reduced privacy [Broersma]. Also, they result in
increased complexity, which might not be possible or desirable in
some implementations (e.g., some embedded devices).
In scenarios in which "Privacy Extensions" are deliberately not used
(possibly for any of the aforementioned reasons), all a host is left
with is the addresses that have been generated using e.g. IEEE
identifiers, and this is yet another case in which it is also vital
that the privacy characteristics of these stable addresses are
improved.
We note that in most (if not all) of those scenarios in which
"Privacy Extensions" are disabled, there is usually no actual desire
to negatively affect user privacy, but rather a desire to simplify
operation of the network (simplify the use of ACLs, logging, etc.).
This document specifies a method to generate interface identifiers
that are stable/constant within each subnet, but that change as hosts
move from one network to another, thus keeping the "stability"
properties of the interface identifiers specified in [RFC4291], while
still mitigating host-scanning attacks and preventing correlation of
the activities of a node as it moves from one network to another.
This document does not update or modify IPv6 StateLess Address Auto-
Configuration (SLAAC) [RFC4862] itself, but rather only specifies an
alternative algorithm to generate Interface IDs. Therefore, the
usual address lifetime properties (as specified in the corresponding
Prefix Information Options) apply when IPv6 addresses are generated
as a result of employing the algorithm specified in this document
with SLAAC [RFC4862]. Additionally, from the point of view of
renumbering, we note that these addresses behave like the traditional
IPv6 addresses (that embed a hardware address) resulting from SLAAC
[RFC4862].
For nodes that currently disable "Privacy extensions" [RFC4941] for
some of the reasons stated above, this mechanism provides stable
privacy-enhanced addresses which may already address most of the
privacy concerns related to addresses that embed IEEE identifiers
[RFC4291]. On the other hand, in scenarios in which "Privacy
Extensions" are employed, implementation of the mechanism described
in this document would mitigate host-scanning attacks and also
mitigate correlation of host activities.
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The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
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2. Design goals
This document specifies a method for selecting interface identifiers
to be used with IPv6 SLAAC, with the following goals:
o The resulting interface identifiers remain constant/stable for
each prefix used with SLAAC within each subnet. That is, the
algorithm generates the same interface identifier when configuring
an address belonging to the same prefix within the same subnet.
o The resulting interface identifiers do not depend on the
underlying hardware (e.g. link-layer address). This means that
e.g. replacing a Network Interface Card (NIC) will not have the
(generally undesirable) effect of changing the IPv6 addresses used
for that network interface.
o The resulting interface identifiers do change when addresses are
configured for different prefixes. That is, if different
autoconfiguration prefixes are used to configure addresses for the
same network interface card, the resulting interface identifiers
must be (statistically) different.
o It must be difficult for an outsider to predict the interface
identifiers that will be generated by the algorithm, even with
knowledge of the interface identifiers generated for configuring
other addresses.
o The aforementioned interface identifiers are meant to be an
alternative to those based on e.g. IEEE identifiers, such as
those specified in [RFC2464].
We note that of use of the algorithm specified in this document is
(to a large extent) orthogonal to the use of "Privacy Extensions"
[RFC4941]. Hosts that do not implement/use "Privacy Extensions"
would have the benefit that they would not be subject to the host-
tracking and host scanning issues discussed in the previous section.
On the other hand, in the case of hosts employing "Privacy
Extensions", the method specified in this document would prevent host
scanning attacks and correlation of node activities across networks
(see Appendix A).
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3. Algorithm specification
IPv6 implementations conforming to this specification MUST generate
interface identifiers using the algorithm specified in this section
in replacement of any other algorithms used for generating "stable"
addresses (such as that specified in [RFC2464]). The aforementioned
algorithm MUST be employed for generating the interface identifiers
for all of the IPv6 addresses configured with SLAAC for a given
interface, including IPv6 link-local addresses.
This means that this document does not formally obsolete or
deprecate any of the existing algorithms to generate Interface IDs
(e.g. such as that specified in [RFC2464]). However, those IPv6
implementations that employ this specification must generate all
of their "stable" addresses as specified in this document.
Implementations conforming to this specification SHOULD provide the
means for a system administrator to enable or disable the use of this
algorithm for generating Interface Identifiers. Implementations
conforming to this specification MAY employ the algorithm specified
in [RFC4941] to generate temporary addresses in addition to the
addresses generated with the algorithm specified in this document.
Unless otherwise noted, all of the parameters included in the
expression below MUST be included when generating an Interface ID.
1. Compute a random (but stable) identifier with the expression:
RID = F(Prefix, Interface_Index, Network_ID, DAD_Counter,
secret_key)
Where:
RID:
Random (but stable) identifier
F():
A pseudorandom function (PRF) that is not computable from the
outside (without knowledge of the secret key). The PRF could
be implemented as a cryptographic hash of the concatenation of
each of the function parameters.
Prefix:
The prefix to be used for SLAAC, as learned from an ICMPv6
Router Advertisement message.
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Interface_Index:
The interface index [RFC3493] [RFC3542] corresponding to this
network interface.
Network_ID:
Some network specific data that identifies the subnet to which
this interface is attached. For example the IEEE 802.11
Service Set Identifier (SSID) corresponding to the network to
which this interface is associated. This parameter is
OPTIONAL.
DAD_Counter:
A counter that is employed to resolve Duplicate Address
Detection (DAD) conflicts. It MUST be initialized to 0, and
incremented by 1 for each new tentative address that is
configured as a result of a DAD conflict. Implementations
that record DAD_Counter in non-volatile memory for each
{Prefix, Interface_Index, Network_ID} tuple MUST initialize
DAD_Counter to the recorded value if such an entry exists in
non-volatile memory). See Section 4 for additional details.
secret_key:
A secret key that is not known by the attacker. The secret
key MUST be initialized at system installation time to a
pseudo-random number (see [RFC4086] for randomness
requirements for security). An implementation MAY provide the
means for the user to change the secret key.
2. The Interface Identifier is finally obtained by taking the
leftmost 64 bits of the RID value computed in the previous step.
The resulting Interface Identifier should be compared against the
list of reserved interface identifiers [IANA-RESERVED-IID], and
against those interface identifiers already employed in an
address of the same network interface and the same network
prefix. In the event that an unacceptable identifier has been
generated, this situation should be handled in the same way as
the case of duplicate addresses (see Section 4).
This document does not require the use of any specific PRF for the
function F() above, since the choice of such PRF is usually a trade-
off between a number of properties (processing requirements, ease of
implementation, possible intellectual property rights, etc.), and
since the best possible choice for F() might be different for
different types of devices (e.g. embedded systems vs. regular
servers) and might possibly change over time.
Note that the result of F() in the algorithm above is no more secure
than the secret key. If an attacker is aware of the PRF that is
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being used by the victim (which we should expect), and the attacker
can obtain enough material (i.e. addresses configured by the victim),
the attacker may simply search the entire secret-key space to find
matches. To protect against this, the secret key should be of a
reasonable length. Key lengths of at least 128 bits should be
adequate. The secret key is initialized at system installation time
to a pseudo-random number, thus allowing this mechanism to be
enabled/used automatically, without user intervention.
Including the SLAAC prefix in the PRF computation causes the
Interface ID to vary across networks that employ different prefixes,
thus mitigating host-tracking attacks and any other attacks that
benefit from predictable Interface IDs (such as host scanning).
Including the optional Network_ID parameter when computing the RID
value above would cause the algorithm to produce a different
Interface Identifier when connecting to different networks, even when
configuring addresses belonging to the same prefix. This means that
a host would employ a different Interface ID as it moves from one
network to another even for IPv6 link-local addresses or Unique Local
Addresses (ULAs).
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4. Resolving Duplicate Address Detection (DAD) conflicts
If as a result of performing Duplicate Address Detection (DAD)
[RFC4862] a host finds that the tentative address generated with the
algorithm specified in Section 3 is a duplicate address, it SHOULD
resolve the address conflict by trying a new tentative address as
follows:
o DAD_Counter is incremented by 1.
o A new Interface ID is generated with the algorithm specified in
Section 3, using the incremented DAD_Counter value.
This procedure may be repeated a number of times until the address
conflict is resolved. We RECOMMEND hosts to try at least
IDGEN_RETRIES (hereby specified as "3") tentative addresses if DAD
fails for successive generated addresses, in the hopes of resolving
the address conflict. We also note that hosts MUST limit the number
of tentative addresses that are tried (rather than indefinitely try a
new tentative address until the conflict is resolved).
In those (unlikely) scenarios in which duplicate addresses are
detected and in which the order in which the conflicting nodes
configure their addresses may vary (e.g., because they may be
bootstrapped in different order), the algorithm specified in this
section for resolving DAD conflicts could lead to addresses that are
not stable within the same subnet. In order to mitigate this
potential problem, nodes MAY record the DAD_Counter value employed
for a specific {Prefix, Interface_Index, Network_ID} tuple in non-
volatile memory, such that the same DAD_Counter value is employed
when configuring an address for the same Prefix and subnet at any
other point in time.
In the event that a DAD conflict cannot be solved (possibly after
trying a number of different addresses), address configuration would
fail. In those scenarios, nodes MUST NOT automatically fall back to
employing other algorithms for generating interface identifiers.
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5. IANA Considerations
There are no IANA registries within this document. The RFC-Editor
can remove this section before publication of this document as an
RFC.
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6. Security Considerations
This document specifies an algorithm for generating interface
identifiers to be used with IPv6 Stateless Address Autoconfiguration
(SLAAC), as an alternative to e.g. interface identifiers that embed
IEEE identifiers (such as those specified in [RFC2464]). When
compared to such identifiers, the identifiers specified in this
document have a number of advantages:
o They prevent trivial host-tracking, since when a host moves from
one network to another the network prefix used for
autoconfiguration and/or the Network ID (e.g., IEEE 802.11 SSID)
will typically change, and hence the resulting interface
identifier will also change (see Appendix A.
o They mitigate address-scanning techniques which leverage
predictable interface identifiers (e.g., known Organizational
Unique Identifiers) [I-D.ietf-opsec-ipv6-host-scanning].
o They result in IPv6 addresses that are independent of the
underlying hardware (i.e. the resulting IPv6 addresses do not
change if a network interface card is replaced).
We note that this algorithm is meant to be an alternative to
interface identifiers such as those specified in [RFC2464], but is
not meant as an alternative to temporary Interface IDs (such as those
specified in [RFC4941]). Clearly, temporary addresses may help to
mitigate the correlation of activities of a node within the same
network, and may also reduce the attack exposure window (since
privacy/temporary addresses are short-lived when compared to the
addresses generated with the method specified in this document). We
note that implementation of this algorithm would still benefit those
hosts employing "Privacy Addresses", since it would mitigate host-
tracking vectors still present when privacy addresses are used (see
Appendix A), and would also mitigate host-scanning techniques that
leverage patterns in IPv6 addresses that embed IEEE identifiers.
Finally, we note that the method described in this document may
mitigate most of the privacy concerns arising from the use of IPv6
addresses that embed IEEE identifiers, without the use of temporary
addresses, thus possibly offering an interesting trade-off for those
scenarios in which the use of temporary addresses is not feasible.
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7. Acknowledgements
The algorithm specified in this document has been inspired by Steven
Bellovin's work ([RFC1948]) in the area of TCP sequence numbers.
The author would like to thank (in alphabetical order) Karl Auer,
Steven Bellovin, Matthias Bethke, Brian Carpenter, Tassos
Chatzithomaoglou, Dominik Elsbroek, Bob Hinden, Christian Huitema,
Ray Hunter, Jong-Hyouk Lee, Michael Richardson, and Ole Troan, for
providing valuable comments on earlier versions of this document.
This document is based on the technical report "Security Assessment
of the Internet Protocol version 6 (IPv6)" [CPNI-IPv6] authored by
Fernando Gont on behalf of the UK Centre for the Protection of
National Infrastructure (CPNI).
Fernando Gont would like to thank CPNI (http://www.cpni.gov.uk) for
their continued support.
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8. References
8.1. Normative References
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
Requirements for Security", BCP 106, RFC 4086, June 2005.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862, September 2007.
[RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy
Extensions for Stateless Address Autoconfiguration in
IPv6", RFC 4941, September 2007.
8.2. Informative References
[RFC1948] Bellovin, S., "Defending Against Sequence Number Attacks",
RFC 1948, May 1996.
[RFC2464] Crawford, M., "Transmission of IPv6 Packets over Ethernet
Networks", RFC 2464, December 1998.
[RFC3493] Gilligan, R., Thomson, S., Bound, J., McCann, J., and W.
Stevens, "Basic Socket Interface Extensions for IPv6",
RFC 3493, February 2003.
[RFC3542] Stevens, W., Thomas, M., Nordmark, E., and T. Jinmei,
"Advanced Sockets Application Program Interface (API) for
IPv6", RFC 3542, May 2003.
[I-D.ietf-opsec-ipv6-host-scanning]
Gont, F. and T. Chown, "Network Reconnaissance in IPv6
Networks", draft-ietf-opsec-ipv6-host-scanning-00 (work in
progress), December 2012.
[IANA-RESERVED-IID]
Reserved IPv6 Interface Identifiers, "http://www.iana.org/
assignments/ipv6-interface-ids/ipv6-interface-ids.xml".
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[Gont-DEEPSEC2011]
Gont, "Results of a Security Assessment of the Internet
Protocol version 6 (IPv6)", DEEPSEC 2011 Conference,
Vienna, Austria, November 2011, .
[Gont-BRUCON2012]
Gont, "Recent Advances in IPv6 Security", BRUCON 2012
Conference, Ghent, Belgium, September 2012, .
[Broersma]
Broersma, R., "IPv6 Everywhere: Living with a Fully IPv6-
enabled environment", Australian IPv6 Summit 2010,
Melbourne, VIC Australia, October 2010,
.
[CPNI-IPv6]
Gont, F., "Security Assessment of the Internet Protocol
version 6 (IPv6)", UK Centre for the Protection of
National Infrastructure, (available on request).
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Appendix A. Privacy issues still present with RFC 4941
This section aims to clarify the motivation of using the method
specified in this document even when privacy/temporary addresses
[RFC4941] are employed. It discusses a (non-exaustive) number of
scenarios in which host privacy is still sacrificed even when
privacy/temporary addresses [RFC4941] are employed, as a result of
employing interface identifiers that are constant across networks
(e.g., those resulting from embedding IEEE identifiers).
A.1. Host tracking
This section describes one possible attack scenario that illustrates
that host-tracking may still be possible when privacy/temporary
addresses [RFC4941] are employed.
A.1.1. Tracking hosts across networks #1
A host configures its stable addresses with the constant
Interface-ID, and runs any application that needs to perform a
server-like function (e.g. a peer-to-peer application). As a result
of that, an attacker/user participating in the same application
(e.g., P2P) would learn the constant Interface-ID used by the host
for that network interface.
Some time later, the same host moves to a completely different
network, and employs the same P2P application, probably even with a
different username. The attacker now interacts with the same host
again, and hence can learn its newly-configured stable address.
Since the interface ID is the same as the one used before, the
attacker can infer that it is communicating with the same device as
before.
This is just *one* possible attack scenario, which should remind us
that one should not disclose more than it is really needed for
achieving a specific goal (and an Interface-ID that is constant
across different networks does exactly that: it discloses more
information than is needed for providing a stable address).
A.1.2. Tracking hosts across networks #2
Once an attacker learns the constant Interface-ID employed by the
victim host for its stable address, the attacker is able to "probe" a
network for the presence of such host at any given network.
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See Appendix A.1.1 for just one example of how an attacker could
learn such value. Other examples include being able to share the
same network segment at some point in time (e.g. a conference
network or any public network), etc.
For example, if an attacker learns that in one network the victim
used the Interface-ID 1111:2222:3333:4444 for its stable addresses,
then he could subsequently probe for the presence of such device in
the network 2011:db8::/64 by sending a probe packet (ICMPv6 Echo
Request, or any other probe packet) to the address 2001:db8::1111:
2222:3333:4444.
A.1.3. Revealing the identity of devices performing server-like
functions
Some devices, such as storage devices or printers, may typically
perform server-like functions and may be usually moved from one
network to another. Such devices are likely to simply disable (or
not even implement) privacy/temporary addresses [RFC4941]. If the
aforementioned devices employ Interface-IDs that are constant across
networks, it would be trivial for an attacker to tell whether the
same device is being used across networks by simply looking at the
Interface ID. Clearly, performing server-like functions should not
imply that a device discloses its identity (i.e., that the attacker
can tell whether it is the same device providing some function in two
different networks, at two different points in time).
The scheme proposed in this document prevents such information
leakage by causing nodes to generate different Interface-IDs when
moving to one network to another, thus mitigating this kind of
privacy attack.
A.2. Address scanning attacks
While it is usually assumed that address-scanning attacks are
unfeasible, an attacker could leverage patterns in IPv6 addresses to
greatly reduce the search space [I-D.ietf-opsec-ipv6-host-scanning]
[Gont-BRUCON2012].
As noted earlier in this document, privacy/temporary addresses do not
eliminate the use of IPv6 addresses that embed IEEE identifiers, and
hence such hosts would still be vulnerable to address-scanning
attacks. The method specified in this document eliminates such
patterns and would thus mitigate the aforementioned address-scanning
attacks.
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Author's Address
Fernando Gont
SI6 Networks / UTN-FRH
Evaristo Carriego 2644
Haedo, Provincia de Buenos Aires 1706
Argentina
Phone: +54 11 4650 8472
Email: fgont@si6networks.com
URI: http://www.si6networks.com
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